“Science,” says our author, “teaches us that, whenever a shock or pressure of any sort is suddenly applied to material of any nature, whether metal, wood, gas, water, air, etc., it is immediately affected in all its parts, from the point of contact to the whole extent of the material, in displacing and replacing the particles of a determinate volume; and the velocity of the movement of the particles of the mass, created by the concussion of shocks or pressure, depends solely (?) upon its elasticity and density. Sound likewise causes motions (?) with every particle of the air, and as far as the motion reaches; so that each particle, with regard to that which lies immediately beyond it, is in a progress of rarefaction during return.”
What is meant by affecting a mass of matter “in all its parts,” by “displacing and replacing the particles of a determinate volume,” we do not precisely understand. That whatever causes motion does it “as far as the motion reaches,” is as unquestionable as any other identical proposition. But that the velocity of the movement of the particles, created by the concussion of shocks, pressure, upon an unconfined elastic fluid, depends solely upon its elasticity and density, we dispute. That pulses “are propagated from a trembling body all around in a spherical manner” may be true, if the air is on all sides equally elastic. Such might be the case with those produced by the vibrations of a bell, when the surrounding air is undisturbed by other causes, and is uniformly elastic at equal distances from it. It would not be strictly true if the initial pulse were made only in a certain direction. “Every impression made on a fluid is propagated every way throughout the fluid, whatever be the direction wherein it is made;” but it is not true that the impressions are equal at equal distances from the initial pulse, irrespective of its direction. This result would presuppose a fluid perfectly elastic; which we never have—and then we might, with equal truth, say that the impressions would be equal at all distances.
Everybody is familiar with the fact that the “transmission of sound,” the pulse which strikes upon the ear to produce the sensation, is affected by currents of air—the direction, force, and velocity of the wind—between the initial pulse and the hearer. How? and how much? directly or indirectly? are questions distinct from the fact itself. The distance through which guns are heard, as well as the loudness of their report, varies with the direction, force, and velocity of the wind; and, in very still air, with the aim of the gun itself, the direction of the initial pulse. For short distances, these differences may be so minute as to escape notice; just as the false proportions of a miniature picture are unobserved until the magnifier displays them. And for longer ranges, they are so small, in contrast with the magnitudes compared, as to seem rather like accidental than legitimate differences. But the difference is not the less real because the reality is less. Words spoken in a faint whisper are clearly heard by a listener immediately before the speaker, when quite inaudible or indistinct to one at an equal distance behind him.
The actual velocities of wind and sound differ so widely that the small fraction by which their relative velocity is denoted is held as proof that the propagation of sound—the pulse—through distances of a few yards or feet, is not affected by currents of air: that there are no differences in the “velocity of sound.” Yet the ear detects them as one of the small differences between discord and harmony in music; distinctness and confusion of speech. In music these differences may be blended by the prolonged intonation of vowel sounds; but in speech, whose distinct significance is due to consonants, “which cannot be sounded without the aid of a vowel,” these differences are fatally evident. The sharp edges of the vocal pulses, which give shape and meaning to vowel sounds, are destroyed alike by a husky voice and a puff of air. What remains is vox et præterea nihil.
It seems to us that some of the many failures in practical acoustics come from considering the air—the material involved—as perfectly elastic. From this it is inferred that sound is not affected by the direction of the initial pulse: that the direction and velocity of the effective pulse are not varied by currents and blasts of air. In short, that the slight inaccuracy of these assumptions will be the actual measurement of resultant error.
Were the purpose only to ascertain the acoustic properties of unadulterated air, varied experiments might eliminate the errors of anomalous results. But when the process is reversed, and we deduce effects from only one among concurrent and conflicting causes, theory is confounded by discordant facts. Theories of sound in purely elastic air might give results approximately realized in practice, if the actual pulses with which we are concerned were given by a flail; but are pregnant of error when the atmosphere is mixed with vicious vapors, and the pulse is a breath of air. Then, the assumption that “pulses of sound” proceed equally in all directions from the initial point, is simply false; and theories based upon it can only complicate the problems to be solved.
Water, as well as air, is a highly elastic fluid, and, if confined and subjected to pressure, the force applied is exerted on all sides of the confined volume. But the effect of a pulse or blow upon a surface of large extent varies with the direction of the force as well as with its power and velocity. We have seen fish swimming near the surface killed or paralyzed by a blow upon the water immediately over them. And we have seen the blow fail of its intended effect solely because it was misdirected. Perhaps the water in the latter case was not perfectly elastic! Neither is the air of churches and public halls, when their atmosphere has yielded a portion of its oxygen, and, in return, is charged with carbonic acid and moist vapors from the breath of crowded assemblies. Carbonic acid gas is heavier by one-half than atmospheric air. It does not, then, always rise toward the ceiling or roof, but remains in solution with impure exhalations; or else, condensed by contact with the colder walls, descends to poison the lower air and impair its elastic force—its power of transmitting the “pulse of sound” to the ear.
We have just come from one of our city churches, where we have had a striking example of this result. The church in question will accommodate (?) about two thousand people. Twenty-five hundred may be crowded into it. At the commencement of the sermon, the preacher’s voice was distinctly audible at points fifty or sixty feet from the pulpit, in spite of reflections of sound—air pulses—from galleries, wooden columns, and the arched ceiling and side-walls, of lath and plaster. Before it was ended, the exhalations of the breathing crowd had so filled the lower half of the “auditorium” that only vowel sounds could be distinguished; and the peroration seemed to consist of spasmodic utterances—scarcely sounds—of a, e, i, o, u. W and y had lost their affinity to vowels, and the rest of the alphabet were no longer consonants, for they were not heard at all.
The acoustic and sanitary problems are here identical—to find a method of preventing an accumulation of foul and inelastic vapors around the breathing and listening congregation, and to give, instead, wholesome air to their lungs, while enabling their ears to hear. And since these poisonous and inelastic gases are specifically heavier than atmospheric air, and must fall to the floor by their own weight, the problem is reduced to providing a practicable way for their escape, and guarding it against counter-currents which might obstruct the passage.